Where are you trying to sign into?
Some things you can try. Do you have access to another WiFi network where you can try? That has worked for several people.
Try a restart.
Restart your iPhone, iPad, or iPod touch *
Try a forced restart. See the section towards the end of the article.
Reset Device. -
If necessary
Hard reset. -
Do a backup and try a factory reset. Then restore from the backup.
Restore - If you see an error when you update or restore your iPhone, iPad, or iPod. *
Backup - Restore your device from an iCloud or iTunes backup. *